This invention relates generally to a screw, and more particularly relates to a drill screw adapted to be used in fastening two or more juxtaposed workpieces of dissimilar materials.
It is a common practice in fastening two workpieces together to provide a pilot hole in the first workpiece of a greater diameter than the major thread diameter of the screw to be utilized in fastening the two workpieces. Such workpieces may comprise two or more panels to be fastened in superimposed or juxtaposed relation, or may comprise a panel to be attached to a frame member or the like. In the absence of such a pilot hole, the threads of the screw might engage the first panel which would increase the driving force required to achieve the final assembly.
Similarly, in many applications a panel of a given material is to be fastened to a frame member of a second material of greater hardness than the material of the panel. For example, panels of wood or other fibrous material are attached to metal frame members in various constructions. In such constructions, it is desirable to provide such a pilot hole in the first workpiece or panel to reduce the amount of torque and "end-load" required to drive the drill screw through the underlying metal frame member. Such reduction in driving torque and "end-load" is reduces worker fatigue and drive tool wear.
Many prior art fasteners have been proposed which utilize one or more wing-like extensions or elements on the shank of the screw to bore a pilot hole in a first workpiece so that the threads of the screw will not engage this workpiece. Moreover, the wings or extensions of such a fastener are designed to break off upon encountering the second metallic workpiece or frame member. Note, for example U.S. Pat. Nos. 3,358,548; 3,699,841 and 3,982,464. While the prior art has proposed various designs for such wing-like extensions on the shank of the screw there is room for yet further improvement.
In particular, it should be recognized that the expense and difficulty of manufacture of such a drill screw should be minimized. Hence, the number and type of machining operations required to form the screw including such wings or extensions should also be held to a minimum of relatively simple machining or forming operations. Preferably in this regard precision machining operations should be avoided in favor of relatively simpler cold forming procedures.